1. Field of the Invention
The present invention relates to a motor controller with an inverter which is capable of controlling the output torque of a motor to be substantially constant even when the rotation speed varies at least in the low speed range.
2. Description of the Prior Art
An induction motor (hereinafter referred to as IM) has advantages of simple structure, low cost, long life and easy maintenance. Because of these merits an IM from small to large capacity has found wide use. However, since no effective method has been available to control the rotation speed efficiently, the application of an IM is relatively limited.
In recent years with various kinds of semiconductor devices available, a static inverter has been developed with which a desired frequency can easily be obtained. By connecting the inverter between the commercial power supply and the IM and changing the output frequency of the inverter, it is possible to control the speed of IM efficiently in wide range of speed.
Most of these inverters not only allow the output frequency to be varied to a desired value but also permit changing of output voltage. On the other hand, the IM is often required to gradually increase its speed to a predetermined value while maintaining the torque constant.
Thus, in the systems where the IM rotation speed is controlled by the inverter, the output voltage of the inverter is varied almost in proportion to the output frequency to control the rotation speed with the torque maintained constant.
An example of such inverter is shown in FIG. 1.
In the figure, reference number 1 denotes a converter or rectifier unit to change the commercial alternating current into a direct current; 2 represents an inverter unit to obtain 3-phase ac current from the dc current; 3 represents an IM; 10 through 20 are diodes; 22 is a smoothing capacitor; 30 through 40 are switching devices such as transistors and gate turn-off thyristors; and 42 through 52 are freewheel diodes.
The diodes 10 through 20 rectify the input 3-phase ac current and supply the rectified dc current to the capacitor 22. Therefore, a smoothed out dc voltage is obtained across the capacitor 22.
The switching devices 30 through 40 are turned on or off by gate signals supplied from a switching control circuit not shown to convert the dc voltage appearing across the capacitor 22 into a 3-phase ac voltage as a supply to the IM 3. By controlling the gate signals it is possible to change the frequency and voltage of the 3-phase ac power, which in turn enables the control of the rotation speed at a desired value while maintaining the torque constant.
FIG. 1 is the case of a pulse width modulation (hereinafter referred to simply as PWM) invertor which provides a desired voltage by chopping the output voltage at the inverter unit 2 since the rectifier unit 1 has no voltage regulation function.
Shown in FIG. 2 is also a conventional inverter in which the rectifier unit 1' is formed of switching devices 10' through 20' such as thyristors or transistors and has a voltage regulation function. Thus the voltage to be applied to the motor 3 is controlled by the rectifier unit 1'. The voltage for the motor 3 can also be controlled by the combined use of the voltage chopping function of the inverter unit 2.
The relation between the inverter output frequency fo and the output voltage Vo is expressed as EQU Vo=a.times.fo+b (1)
where a and b are constants. By controlling such that the above equation (1) is always satisfied, the rotation speed of IM can be controlled at a desired speed while maintaining the torque constant. Therefore, if the constants a and b in the equation (1) are set at required values in accordance with the maximum output frequency fo.sub.max or the upper limit of the variable range of the output frequency fo, the rotation speed of IM can be controlled in the range from a reasonably low speed to the rated speed with the torque maintained constant.
The conventional inverters, however, have some drawbacks. That is, when used in such a condition that the maximum output frequency differs from the set value, the inverters cannot fully demonstrate their performance or an overload will result. Hence when the input power supply frequency is different, the conventional inverter cannot be used, in other words it has poor interchangeability.
To meet the requirement of constant torque, the constants a and b of equation (1) must be changed according to the maximum output frequency. For instance, as shown in FIG. 3, the characteristic A for the maximum output frequency of 50 Hz must be made different from the characteristic B of the maximum output frequency of 60 Hz. The output voltage Vo of FIG. 3 is so determined that the voltage at the maximum output frequency is 100% of the rated voltage.
There are different power supply systems in different regions of the world and the power is supplied at 50 Hz in some regions and at 60 Hz in other regions. When the inverter set for the maximum output frequency of 60 Hz is used in the region where the power with 50 Hz is supplied, the torque of the IM does not reach the rated value of 100% as indicated by the point P.sub.1 along the characteristic B. Conversely, when the inverter for 50 Hz is used in the region where 60 Hz is used, the torque of IM exceeds the rated value of 100% as indicated by the point P.sub.2 along the characteristic A resulting in the overload of the inverter. In addition, the maximum speed may not reach the rated rotation speed.
As can be seen in the foregoing, the conventional motor controller has poor interchangeability, i.e., when the input frequency does not match it cannot be used.